Receiver with chip-level equalisation

Abstract

The present invention relates to receiver apparatuses and methods of controlling weight adaptation in a receiver of a code multiplex telecommunications system with orthogonal spreading codes, wherein received discrete time signal samples are chip-level filtered by using a first equalizing step. Additionally, the received discrete time signal samples are delayed by a time period corresponding to a data symbol and used in a second equalizing step. Symbol estimates obtained from the first equalizing step are non-linearly filtered and used as a desired response for the second equalizing step in the following symbol period, wherein equalizer weights adapted in the second equalizing step are used for the first equalizing step. Alternatively, the second equalizing step may be dispensed with and weight adaptation may be incorporated in a single equalizing step. As an additional or alternative option, a hybrid equalizer architecture may be provided, where the above two-step equalization is used during an active phase where a channel is allocated, while another weight updating scheme is used during an inactive phase where no channel is assigned. Thereby, detrimental effects of interference power can be reduced at low increase in complexity.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a receiver apparatus and method of controlling weight adaptation in a receiver of a code multiplex telecommunications system with orthogonal spreading codes. As an example, the present invention relates to a receiver apparatus and weight adaptation method for a High Speed Downlink Packet Data Access (HSDPA) system as introduced in the Universal Mobile Telecommunications System (UMTS) Release-5 standard.BACKGROUND OF THE INVENTION[0002]Code Division Multiple Access (CDMA) systems are based on a digital wideband spread spectrum technology in which multiple independent user signals are transmitted across an allocated segment of the available radio spectrum. In CDMA, each user signal comprises a different orthogonal code and a pseudo random binary sequence that modulates a carrier, thereby spreading the spectrum of the waveform and thus allowing a large number of user signals to share the same frequency spectrum. The user sign...

AI Technical Summary

Benefits of technology

[0021]It is therefore an object of the present invention to provide an improved receiver end weight adaptation control method, by means of which defects of interference power can be reduced while complexity increase can be kept small. According to a first aspect, this object is achieved by a receiver apparatus and a weight adaptation control method respectively. According to an embodiment, the receiver apparatus comprises a filtering branch with first equaliser means for receiving discrete time signal samples and for chip-level filtering said received signal samples; feedback means for non-linear filtering symbol estimates obtained in said filtering branch and feeding the filtered symbol estimates back to the first equaliser means; adaptation means for receiving said received signal samples via a delay means arranged for delaying said received signal samples by a time period corresponding to a data symbol, wherein said filtered symbol estimates are used by the adaptation means as desired response for adapting equaliser weights of the first equaliser means in the following symbol period. According to an embodiment, the weight adaptation control method comprises receiving discrete time signal samples and chip-level filtering said received signal samples by using a first equalising step; delaying said received discrete time signal samples by a time period corresponding to a data symbol and using said delayed time signal samples in an adaptation function for said first equalising step; and applying a non-linear filtering operation to symbol estimates obtained from said first equalising step and using the filtered symbol estimates as a desired response for said adaptation function for adapting equaliser weights for said first equalizing step in the following symbol period.

[0022]Accordingly, the delay introduced in the adaptation branch is fixed and reduced to only one symbol period of 16 chips. Therefore, filter weights can be copied from the equaliser of the adaptation branch to the equaliser of the filtering branch without requiring any prediction. Moreover, the non-linear filtering of the symbol estimates can be based on knowledge of the specific channel codes, so that a more robust method and system can be achieved.

[0023]Additionally, in the solution wherein the first equaliser means comprises the adaptation means, only one equalising function or unit is necessary, which leads to a considerable reduction of complexity, overhead, and power consumption.

Problems solved by technology

Other user signals whose codes do not match are not de-spread and as such contribute to system noise.

In CDMA systems in general, however, due to multipath propagation and frequency-selective fading, orthogonality between the various users waveforms is degraded and multiple access interference impairs the performance of the receiver.

This loss of orthogonality may cause inter-code interference (also known as multi-user interference or multi-access interference), inter-chip interference and inter-symbol interference in the symbol estimates.

However, the possibility to implement MUDs or ICs in mobile stations is limited due to their high complexity and due to the fact that transmission parameters of all users are usually not known.

However, when small spreading factors are used to achieve high data rates like, for example, in HSDPA systems, performance of the Rake receiver decreases due to the fact that the multipath interference becomes significant and the correlation characteristics of the spreading sequences are destroyed.

Since it will not be possible to make reliable symbol estimates without a properly adjusted equaliser, the DD mode is only engaged after a preceding pilot-trained mode has converged.

The above adaptive equaliser described by Schniter et al. does not provide an optimal solution for high speed channels as provided in the HSDPA system.

Due to the large delay introduced in the adaptation branch of the delayed equaliser, the adapted filter weights or taps cannot be directly used at the upper branch filtering operation of the tentative equaliser.

Moreover, a substantial delay which corresponds to the maximum active spreading factor in the system is introduced and can even be 512 chips in some cases.

This is however a very complicated process and hence not easy to implement.

Even when implemented, problems of false detection, missing detection and wrongly estimated amplitudes may still occur.

Moreover, de-spreading is done with each active code independently at various levels in the OVSF trees, which leads to a high computational complexity.

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